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Despite the high virological response rates achieved with current directly acting antiviral agents (DAAs) against hepatitis C virus (HCV), around 2% to 5% of treated patients do not achieve a sustained viral response. The identification of amino acid substitutions associated with treatment failure requires analytical designs, such as subtype-specific ultradeep sequencing (UDS) methods, for HCV characterization and patient management. Using this procedure, we have identified six highly represented amino acid substitutions (HRSs) in NS5A and NS5B of HCV, which are not bona fide resistance-associated substitutions (RAS), from 220 patients who failed therapy. They were present frequently in basal and posttreatment virus of patients who failed different DAA-based therapies. Contrary to several RAS, HRSs belong to the acceptable subset of substitutions according to the PAM250 replacement matrix. Their mutant frequency, measured by the number of deep sequencing reads within the HCV quasispecies that encode the relevant substitutions, ranged between 90% and 100% in most cases. They also have limited predicted disruptive effects on the three-dimensional structures of the proteins harboring them. Possible mechanisms of HRS origin and dominance, as well as their potential predictive value for treatment response, are discussed., The work at CBMSO was supported by grants SAF2014-52400-R from Ministerio de Economía y Competitividad (MINECO), SAF2017-87846-R and BFU2017-91384-EXP from Ministerio de Ciencia, Innovación y Universidades (MICIU), PI18/00210 from Instituto de Salud Carlos III, S2013/ABI-2906 (PLATESA from Comunidad de Madrid/FEDER), and S2018/BAA-4370 (PLATESA2 from Comunidad de Madrid/FEDER). C.P. is supported by the Miguel Servet program of the Instituto de Salud Carlos III (CP14/00121 and CPII19/00001), cofinanced by the European Regional Development Fund (ERDF). CIBERehd (Centro de Investigación en Red de Enfermedades Hepáticas y Digestivas) is funded by Instituto de Salud Carlos III. Institutional grants from the Fundación Ramón Areces and Banco Santander to the CBMSO are also acknowledged. The team at CBMSO belongs to the Global Virus Network (GVN). The work in Barcelona was supported by Instituto de Salud Carlos III, cofinanced by the European Regional Development Fund (ERDF) grant number PI19/00301 and by the Centro para el Desarrollo Tecnológico Industrial (CDTI) from the MICIU, grant number IDI-20151125. Work at CAB was supported by MINECO grant BIO2016-79618R and PID2019-104903RB-I00 (funded by the EU under the FEDER program) and by the Spanish State research agency (AEI) through project number MDM-2017-0737 Unidad de Excelencia “María de Maeztu”-Centro de Astrobiología (CSIC-INTA). Work at IBMB was supported by MICIN grant BIO2017-83906-P (funded by the EU under the FEDER program). C.G.-C. is supported by predoctoral contract PRE2018-083422 from MICIU. B.M.-G. is supported by predoctoral contract PFIS FI19/00119 from Instituto de Salud Carlos III (Ministerio de Sanidad y Consumo), cofinanced by Fondo Social Europeo (FSE).

The Picornaviridae is a large family of positive-sense RNA viruses that contains numerous human and animal pathogens, including foot-and-mouth disease virus (FMDV). The picornavirus replication complex comprises a coordinated network of proteinprotein and protein-RNA interactions involving multiple viral and host-cellular factors. Many of the proteins within the complex possess multiple roles in viral RNA replication, some of which can be provided in trans (i.e., via expression from a separate RNA molecule), while others are required in cis (i.e., expressed from the template RNA molecule). In vitro studies have suggested that multiple copies of the RNA-dependent RNA polymerase (RdRp) 3D are involved in the viral replication complex. However, it is not clear whether all these molecules are catalytically active or what other function(s) they provide. In this study, we aimed to distinguish between catalytically active 3D molecules and those that build a replication complex. We report a novel nonenzymatic cis-acting function of 3D that is essential for viral-genome replication. Using an FMDV replicon in complementation experiments, our data demonstrate that this cis-acting role of 3D is distinct from the catalytic activity, which is predominantly trans acting. Immunofluorescence studies suggest that both cis-and trans-acting 3D molecules localize to the same cellular compartment. However, our genetic and structural data suggest that 3D interacts in cis with RNA stem-loops that are essential for viral RNA replication. This study identifies a previously undescribed aspect of picornavirus replication complex structure-function and an important methodology for probing such interactions further., This work, including the efforts of Eleni-Anna Loundras and Joseph Ward, was funded by Biotechnology and Biological Sciences Research Council (BBSRC) (BB/F01614X/1). This work, including the efforts of Morgan R. Herod, David J. Rowlands, and Nicola J. Stonehouse, was funded by Biotechnology and Biological Sciences Research Council (BBSRC) (BB/K003801/1). This work, including the efforts of Nuria Verdaguer, was funded by Ministerio de Economía y Competitividad (MINECO) (BIO2014-54588-P). This work, including the efforts of Cristina Ferrer-Orta, was funded by Ministerio de Economía y Competitividad (MINECO) (MDM-2014-0435).

Viral RNA-dependent RNA polymerases (RdRps) are major determinants of high mutation rates and generation of mutant spectra that mediate RNA virus adaptability. The RdRp of the picornavirus foot-and-mouth disease virus (FMDV), termed 3D, is a multifunctional protein that includes a nuclear localization signal (NLS) in its N-terminal region. Previous studies documented that some amino acid substitutions within the NLS altered nucleotide recognition and enhanced the incorporation of the mutagenic purine analogue ribavirin in viral RNA, but the mutants tested were not viable and their response to lethal mutagenesis could not be studied. Here we demonstrate that NLS amino acid substitution M16A of FMDV serotype C does not affect infectious virus production but accelerates ribavirin-mediated virus extinction. The mutant 3D displays polymerase activity, RNA binding, and copying processivity that are similar to those of the wild-type enzyme but shows increased ribavirin-triphosphate incorporation. Crystal structures of the mutant 3D in the apo and RNA-bound forms reveal an expansion of the template entry channel due to the replacement of the bulky Met by Ala. This is a major difference with other 3D mutants with altered nucleotide analogue recognition. Remarkably, two distinct loop β9-α11 conformations distinguish 3Ds that exhibit higher or lower ribavirin incorporation than the wild-type enzyme. This difference identifies a specific molecular determinant of ribavirin sensitivity of FMDV. Comparison of several polymerase mutants indicates that different domains of the molecule can modify nucleotide recognition and response to lethal mutagenesis. The connection of this observation with current views on quasispecies adaptability is discussed., Work in Madrid was supported by grants BFU2011-23604, SAF2014-52400-R, and AGL2014-52395-C2-1-R from the Spanish MINECO and S2013/ABI-2906 (PLATESA from Comunidad de Madrid/FEDER). CIBERehd (Centro de Investigación en Red de Enfermedades Hepáticas y Digestivas) is funded by Instituto de Salud Carlos III. Work in Barcelona was supported by grants BIO2017-83906-P and Maria de Maeztu Unit of Excellence MDM-2014-0435, from the Spanish MINECO. X-ray data were collected at ALBA Synchrotron (Cerdanyola de Valles, Spain) XALOC beamline with the collaboration of ALBA staff and at ESRF beamline ID29 (Grenoble, France). Financial support was provided by ALBA and ESRF. The kinetic studies at the University of Missouri were partially supported by grant AI076119 (National Institutes of Health, USA) to S.G.S. C.P. is supported by the Miguel Servet program of the Instituto de Salud Carlos III (CP14/ 00121) cofinanced by the European Regional Development Fund (ERDF). Institutional grants from the Fundación Ramón Areces and Banco Santander to the CBMSO are also acknowledged.

© 2015, American Society for Microbiology. The N-terminal region of the foot-and-mouth disease virus (FMDV) 3D polymerase contains the sequence MRKTKLAPT (residues 16 to 24) that acts as a nuclear localization signal. A previous study showed that substitutions K18E and K20E diminished the transport to the nucleus of 3D and 3CD and severely impaired virus infectivity. These residues have also been implicated in template binding, as seen in the crystal structures of different 3D-RNA elongation complexes. Here, we report the biochemical and structural characterization of different mutant polymerases harboring substitutions at residues 18 and 20, in particular, K18E, K18A, K20E, K20A, and the double mutant K18A K20A (KAKA). All mutant enzymes exhibit low RNA binding activity, low processivity, and alterations in nucleotide recognition, including increased incorporation of ribavirin monophosphate (RMP) relative to the incorporation of cognate nucleotides compared with the wild-type enzyme. The structural analysis shows an unprecedented flexibility of the 3D mutant polymerases, including both global rearrangements of the closed-hand architecture and local conformational changes at loop β9-α11 (within the polymerase motif B) and at the template-binding channel. Specifically, in 3D bound to RNA, both K18E and K20E induced the opening of new pockets in the template channel where the downstream templating nucleotide at position +2 binds. The comparisons of free and RNA-bound enzymes suggest that the structural rearrangements may occur in a concerted mode to regulate RNA replication, processivity, and fidelity. Thus, the Nterminal region of FMDV 3D that acts as a nuclear localization signal (NLS) and in template binding is also involved in nucleotide recognition and can affect the incorporation of nucleotide analogues. IMPORTANCE : The study documents multifunctionality of a nuclear localization signal (NLS) located at the N-terminal region of the foot-andmouth disease viral polymerase (3D). Amino acid substitutions at this polymerase region can impair the transport of 3D to the nucleus, reduce 3D binding to RNA, and alter the relative incorporation of standard nucleoside monophosphate versus ribavirin monophosphate. Structural data reveal that the conformational changes in this region, forming part of the template channel entry, would be involved in nucleotide discrimination. The results have implications for the understanding of viral polymerase function and for lethal mutagenesis mechanisms., Research in Barcelona was supported by grant BIO2011-24333 from the Spanish MINECO and by SILVER Cooperation project GA number 260644 of the European Union, Seventh Framework Program. Work in Madrid was supported by grants BFU2011-23604 and BIO2011-24351 from MINECO, PLATESA-S2013/ABI-2906 from CAM, and Fundación Ramón Areces. Financial support was provided by the ESRF and Alba. C.F.-O. is a recipient of a JAE postdoctoral contract supported by the Fondo Social Europeo

Viral pathogenesis is not alien to the evolutionary history of a virus. Picornaviruses, simply by the fact of sharing a phylogenetic position, need not be associated with similar diseases, reflecting that the nature of the interactions with their host organisms may depend in a subtle manner on minimal genetic change of the virus. Picornaviruses have served to establish core concepts in the understanding of viruses as mutated collectivities and in establishing the relevance of quasispecies for viral pathogenesis. The adaptive potential of RNA viruses is also manifested in the response to selective agents administered to inhibit their replication. High mutation rates result in the almost-systematic selection of viral mutants resistant to antiviral inhibitors, either because resistant mutants are present in mutant spectra or because they are rapidly generated during viral replication. The participation of interfering genomes in virus extinction constitutes the basis of the lethal defection model of virus extinction by enhanced mutagenesis. The initial experiments to test the validity for RNA viruses of the error threshold concept consisted of documenting an adverse effect on viral infectivity as a result of increasing the mutation rate of poliovirus (PV) and vesicular stomatitis virus by chemical mutagens and base and nucleoside analogues added during viral RNA replication. Genetic modifications upon extensive passage of FMDV in BHK- 21 cells included genomes with internal in-frame deletions that were infectious by complementation in the absence of standard, wildtype genomes.